Tertiary alkyl ethers, particularly methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) are of high interest for improving the gasoline qualities.
Thus, the use of methyl tert-butyl ether (MTBE),in view of its antiknock properties, improves the quality of commercial gasolines, resulting in a higher octane number than that obtained by addition of methanol, one of the best additives. In addition, methyl tert-butyl ether (MTBE) has a calorific value higher than that of methanol: 8,395 kcal/kg (35,091 kJoule/kg) for MTBE, as compared with 4,764 kcal/kg (19,914 kJoule/kg) for methanol (as an average the calorific value of a primium gasoline is 10,200 kcal/kg, i.e 42,636 kJoule/kg). Moreover, the use of MTBE does not result in demixion difficulties in the presence of water, as for methanol. Finally, the solubility in water of MTBE being considerably higher than that of water in hydrocarbons, the addition of MTBE improves the tolerance to water of motor-fuels.
It is known to prepare tertiary alkyl ethers, particularly methyl tert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) which are the most conventional ethers, by reacting an iso-olefin, generally contained in a hydrocarbon fraction, with an alcohol, for example methanol, in the presence of an acid catalyst, for example sulfuric acid, hydrofluoric acid, aluminum chloride or boron fluoride, or in the presence of carbonaceous materials containing --SO.sub.3 H groups, for example sulfonated coals, sulfonated phenol-formaldehyde resins, sulfonated coumarone-indene polymers or preferably sulfonated polystyrene-divinylbenzene resins.
It has been known for a long time that the reaction between methanol and tertiary olefins is a balanced reaction and therefore it is difficult to obtain high conversion rates. The equilibrium is less displaced in favor of ether formation so that the molecular weight of the iso-olefin is higher. Thus, for iso-amylenes, the conversion rate is limited to 65-75% when the use of a too large methanol excess is to be avoided. To obtain acceptable conversion rates of iso-olefins, particularly of iso-amylenes, a very large methanol excess with respect to the stoichiometry must be used. Consequently, the methanol, amount contained in the reaction mixture is too large to be easily removed in a conventional manner, such as by azeotropic distillation with hydrocarbons and recycling to the reactor, as disclosed in the French patent 2 411 881. According to U.S. Pat. No. 4,204,077, methanol could then be removed by extraction with a solvent such as ethylene glycol.
In these conventional techniques, the tertiary alkyl ether formed by reaction of an alcohol with an iso-olefin contained in a hydrocarbon mixture, is obtained within a mixture of unconverted hydrocarbons and occasionally of unconverted alcohol. After completion of the etherification reaction, generally in at least two reactors, the tertiary alkyl ether must be separated from the other constituents in several distillation columns, while simultaneously removing the maximum alcohol amount with the minimum loss of ether.
Thus, the use of several reactors and distillation columns for manufacturing and separating the tertiary alkyl ether increases the investment and operating costs without giving a high ether yield.
A method has been proposed for solving the relevant problems: it involves in the reactive distillation (or catalytic distillation), in which the etherification reaction with a catalyst and the distillation for separating the tertiary alkyl ether, as it is formed, from the other unconverted constituents are performed in the same enclosure (U.S. Pat. No. 3,629,478 EP-B 8 860, FR 2 503 700).
According to U.S. Pat. No. 3,629,478, the catalyst is placed in bulk in discharge gutters (or downcomers) of distillation sieve trays: according to this patent only the descending liquid phase is in contact with the catalyst, the vapor phase rising through the perforations of each distillation tray. In fact, due to the reaction exothermicity, the formation of a vapor phase at contact with the catalyst is unavoidable and gives rise to a hydrodynamic problem: as a matter of fact, it will be very difficult, if not impossible, for a mixed phase consisting of the liquid plus the vapor formed by the reaction heat, i.e a light phase, to descend through the discharge gutters, in view of the high resistance to its passage due to the small section of said gutters and to the catalyst contained therein.
The European patent 8 860 proposes to feed an isobutene and methanol-containing mixture into a distillation column filled with a catalyst convenient for producing methyl tert-butyl ether (MTBE), wherein the catalyst also acts as packing for the distillation, thus forming MTBE and simultaneously separating C.sub.4 constituents.
Although the process disclosed in this patent already represents an important technical advance in the field of reactive distillation, it is further substantially improved according to the invention, as a result, in particular, of a more important distillation operation.
French patent 2 503 700 proposes the use of a series of catalytic steps with ascending vapor-liquid flow through each catalyst bed with the catalyst being embedded. But the distillation effect is not as important as expected. Moreover, a hydrodynamic problem may arise : as a matter of fact, in view of the gravity effect, it is not easy for the fluid to flow upwardly through each catalyst bed.